1. Field of the Invention
The present invention relates to magnetic random access memory (MRAM) devices and, more particularly, to magnetic tunnel junction structures having bended tips at both ends thereof, MRAM cells employing the same, and photo masks used in formation thereof.
2. Description of the Related Art
MRAM devices have been widely used as nonvolatile memory devices, which can be operated at a low voltage and at a high speed. In a unit cell of the MRAM devices, data is stored in a magnetic tunnel junction (MTJ) structure of a magnetic resistor. The MTJ structure includes first and second ferromagnetic layers and a tunneling insulation layer interposed therebetween. Magnetic polarization of the first ferromagnetic layer referred to as a free layer may be changed by an external magnetic field that crosses the MTJ structure. The external magnetic field may be induced by a current that flows around the MTJ structure, and the magnetic polarization of the free layer may be parallel or anti-parallel to the fixed magnetic polarization of the second ferromagnetic layer referred to as a pinned layer. Current for creating the magnetic field flows through conductive layers called a digit line and a bit line, which are disposed around the MTJ structure.
According to spintronics based on quantum mechanics, when magnetic spins in the free layer and the pinned layer are arrayed to be parallel to each other, a tunneling current passing through the MTJ structure exhibits a maximum value. On the other hand, when the magnetic spins in the free layer and the pinned layer are arrayed to be anti-parallel to each other, the tunneling current passing through the MTJ structure has a minimum value. Accordingly, data of the MRAM cell can be determined according to the direction of the magnetic spins in the free layer.
Most of the MTJ structures have a rectangular shape or an ellipse shape when viewed from a plan view. This is because the magnetic spins in the free layer have a stable state when the magnetic spins in the free layer are parallel to a longitudinal direction of the free layer.
The MRAM device includes a plurality of MTJ structures. The plurality of MTJ structures may exhibit non-uniform switching characteristics according to a fabrication process thereof. In this case, external magnetic fields for storing desired data in the MTJ structures may be different from one another. Accordingly, the more switching characteristics of the MTJ structures are non-uniform, the more the writing margin of the MRAM device is reduced. In particular, when the MTJ structures are scaled down for high integration density, the writing margin may be significantly reduced.
In the event that the MTJ structure employs a pinned layer composed of a single ferromagnetic layer, a hysteresis curve of the MTJ structure may be shifted due to a parasitic magnetic field by fixed magnetic spins in the pinned layer. That is, even though an external magnetic field is not applied to the MTJ structure, the magnetic spins in the free layer may be under the influence of the parasitic magnetic filed which is due to the fixed magnetic spins in the pinned layer. Accordingly, an absolute value of a first external magnetic field to array the magnetic spins in the free layer to be parallel with the fixed magnetic spins in the pinned layer may be different from that of a second external magnetic field to array the magnetic spins in the free layer to be anti-parallel with the fixed magnetic spins in the pinned layer. In order to solve the above-mentioned problem, a synthetic anti-ferromagnetic (SAF) layer has been widely used as the pinned layer.
In addition, the free layer may also employ the synthetic anti-ferromagnetic layer. In this case, the free layer composed of the synthetic anti-ferromagnetic layer exhibits better thermal stability and less magnetic polarization deviation as compared to the free layer composed of the single ferromagnetic layer.
A magnetic tunnel junction (MTJ) structure having a pinned layer and a free layer, which are made of the synthetic anti-ferromagnetic layer, is described in U.S. Pat. No. 6,531,723 B1 to Engel et al., entitled “Magnetoresistance Random Access Memory for Improved Scalability”. Nevertheless, in the event that the MTJ structure according to Engel et al. has a rectangular or ellipse shape when viewed from a plan view, there may be some limitations in improving the writing margin.
Moreover, MRAM devices employing MTJ structures with different shapes from the rectangular and ellipse shapes are described in U.S. Patent Publication No. US 2003/0169147 A1 to Higo, entitled “Magnetoresistive Effect Element and Magnetic Memory Device.” According to Higo, a vertical write electrode passes through a region between a pair of C-shaped free layers, which face each other. The write electrode is electrically connected to a pinned layer that is insulated from the free layers and located below the free layers. In addition, read lines electrically connected to the free layers are provided at both sides of the write electrode. Accordingly, the MTJ structure by Higo may require a complex manufacturing process. Further, the MTJ structure according to Higo is switched by only a vertical current that flows through the vertical write electrode. Thus, a large writing current may be required to change data stored in the MTJ structure. This may lead to an increase of power consumption during a writing operation.